Method of manufacturing domain inverted crystal

ABSTRACT

A first electrode is partially contacted with a domain to be polarization-inverted  2  on one plate face of a nonlinear optical crystal substrate  1 , a second electrode is contacted with the other plate face of the substrate, and a polarization inversion voltage is applied between the both electrodes. At this time, the electrode is so formed that the contact area of the first electrode  3  with the plate face satisfies particular conditions, and the domain to be polarization-inverted is entirely or partially polarization-inverted by the application of a polarization inversion voltage. The aforementioned particular conditions are that each contact area  3  is dot-like so that plural contact areas  3  can be present independently within individual domains to be polarization-inverted  2 , and individual dot-like contact areas have an area of 0.00785 μm  2 -7850 μm 2  and a shape included in a circle having a diameter of 100 μm. As a result, a polarization inverted crystal having high quality can be obtained more easily.

TECHNICAL FIELD

The present invention relates to a production method of a polarizationinverted crystal (namely, a nonlinear optical crystal wherein a periodicpolarization inverted structure (periodically domain-inverted structure)is formed).

BACKGROUND ART

It is known that a ferro-electric crystal (particularly a nonlinearoptical crystal) can be utilized as a wavelength converting element byperiodically inverting the direction of spontaneous polarization forevery specific domain (e.g., JP-A-6-110095).

A structure wherein the direction of spontaneous polarization isinverted for every specific domain as mentioned above is a periodicpolarization inverted structure (hereinafter a polarization invertedstructure), and a nonlinear optical crystal partly or entirely impartedwith the polarization inverted structure is a polarization invertedcrystal.

A polarization inverted crystal can be used as an element for variouswavelength conversions, such as Second Harmonic Generation (SHG),Optical Parametric Oscillation (OPO), Difference Frequency Generation(DFG), Sum Frequency Generation (SFG) and the like, by changing theperiod of its polarization inverted structure. Therefore, application ofa wavelength converting element using a polarization inverted crystal tothe fields of optical communication, optical information processing, gasdetection and the like has been studied, since it can realize a widerange of wavelength conversion from blue visible bounds to infraredbounds.

FIGS. 15(a) and 15(b) each show an example of a polarization invertedcrystal, wherein an inverted domain R1 with inverted polarizationdirection and a non-inverted domain N1 with the original crystalpolarization direction are alternately arranged at a predeterminedperiod in a crystal substrate 11 to form a stripe pattern. In FIG.15(a), a polarization inverted structure is formed on the surface layerof a nonlinear optical crystal substrate (hereinafter “crystalsubstrate”) 11 and a waveguide 12 is formed to cross the structure. InFIG. 15(b), a polarization inverted structure is formed in the entiretyof a crystal substrate to give what is called a bulk type wherein theoptical path is not limited. When an input light L10 to bewavelength-converted alternately passes a non-inverted domain and aninverted domain, an output light L20 is produced and emitted, whichoutput light L20 is wavelength-converted by a nonlinear optical effectof crystal and a quasi-phase matching of a polarization invertedstructure.

As a preferable production method of conventional polarization invertedcrystals, the methods described in U.S. Pat. No. 5,800,767 (FIG. 16) andU.S. Pat. No. 5,193,023 can be mentioned. According to this method, aresist pattern 22 made of an insulator is so formed on one plate face(upper surface of the substrate in this Figure; +z-plane) of a crystalsubstrate (z plate) 11 that only the domain to be polarization invertedis exposed, and an electrode layer 23 made of a metal electrode materialis uniformly formed thereon, as shown in FIG. 16. As a result, theelectrode layer 23 contacts only with the domain to be polarizationinverted (hereinafter an electrode of the contacted part is to bereferred to as an “upper electrode”) and acts as an electrode forinversion. In the example shown in this Figure, a liquid electrode 24 isfurther contacted on the electrode layer 23, which in turn enablesapplication of a uniform voltage to the entire surface of the electrodelayer 23.

A liquid electrode (liquid electrolyte) 31 contacts the entire surfaceof the other plate face (lower surface of the substrate in this Figure;−z-plane) in FIG. 16. A container and the like for contacting with aliquid electrode is not shown. Both electrodes are each connected withan electric power-supply unit S10 (not shown) for applying an inversionvoltage, and when a polarization inversion voltage is applied betweenboth electrodes (+electric potential is applied to +Z-plane and-electric potential is applied to −Z-plane), the spontaneouspolarization direction of crystal is inverted to give a polarizationinverted crystal. In FIG. 16, an electric field generated bypolarization inversion voltage is shown with an arrow.

In the above-mentioned polarization inversion method, when only the areabeneath an upper electrode 23 (hatched part 11 a in the oppositedirection) is polarization-inverted, whose ideal state is shown in FIG.17(a), the quality of the polarization inverted structure depends solelyon the precision of formation of the electrode pattern and no particularproblem is produced.

However, in an actual polarization inversion processing, as shown inFIG. 17(b), a polarization-inverted domain is formed not only in adomain beneath an upper electrode 23 but extends from the upperelectrode 23, like a domain 11 b in this Figure. It is difficult topositively suppress such overflow from the polarization-inverted domain,and a countermeasure such as setting the width of an upper electrodenarrower in anticipation of the amount of overflow and the like has beentaken. In other words, the size of a polarization-inverted domaindepends on the results of the inversion step and production of apolarization inverted structure having a high precision period has beendifficult. To inhibit overflow from the polarization-inverted domain,strict control of the applied voltage is required, and repeatedproduction of polarization inverted crystals having similar quality hasnot been easy.

Taking note of the contact area between a crystal substrate face and anupper electrode, a phenomenon wherein polarization inversion proceedsfrom the corner of the circumference of a contact area due to anedge-effect by which electric fields gather to the edge having a shapeof an outer circumference of the upper electrode is seen, which becomesa problem.

To be specific, as shown in FIG. 18, the arrangement pattern of theupper electrode to form a polarization inverted structure is astripe-like pattern, wherein individual electrode shape (=shape ofcontact area) 12 becomes a band as shown with a dashed line. When apolarization inversion voltage is applied to such a band electrode, theedge-effect causes polarization inversion first in the both ends of theband, and when application of the voltage is continued, the inversionproceeds from the both ends toward the center, and finally, the entirearea of the band is inverted.

However, the inversion actually proceeds not only from the both endstoward the center but also toward the period direction (direction towardthe neighboring electrode). Thus, when the whole inversion is completed,the both ends inverted first have spread too much in the perioddirection (hatched part 13 in this Figure).

Consequently, problems including a nonuniformly inverted structure(particularly, inversion ratio) in the crystal substrate surface, andjoining of the neighboring inverted domains to cause failure infunctioning as a polarization inverted structure occur. In addition,since the voltage applying conditions vary depending on the polarizationinversion period and the size of electrode area, specific conditionsneed to be determined for each shape of the product.

DISCLOSURE OF THE INVENTION

The problem of the present invention is to provide a production methodof a polarization inverted crystal that can solve the above-mentionedproblems and can easily afford a polarization inverted crystal havinghigh quality.

The present invention is characterized by the following.

(1) A production method of a polarization inverted crystal, whichcomprises a step of bringing a first electrode into partial contact withdomain(s) to be polarization-inverted, which domains are present in thenumber of not less than 1 in one plate face of a nonlinear opticalcrystal substrate, bringing a second electrode into contact with theother plate face of the substrate, and applying a polarization inversionvoltage between the both electrodes,

wherein, in the aforementioned step, the electrodes are so formed thatthe contact area of the first electrode relative to the plate facesatisfies the conditions of the following (A), and the domain to bepolarization-inverted is entirely or partially polarization-inverted bythe application of a polarization inversion voltage:

(A) respective contact areas are dispersed like dots in individualdomains to be polarization-inverted such that plural contact areas areindependently present, and individual dot-like contact areas have anarea of 0.00785 μm²-7850 μm² and a shape included in a circle with adiameter of 100 μm.

(2) The production method of the above-mentioned (1), wherein the domainto be polarization-inverted is entirely polarization-inverted by theapplication of a polarization inversion voltage.

(3) The production method of the above-mentioned (1), wherein the domainto be polarization-inverted is partially polarization-inverted by theapplication of a polarization inversion voltage and the partialpolarization inversion is any of the modes of the following (i)-(iv):

(i) a mode wherein an area about the same as the contact area of thefirst electrode is polarization-inverted,

(ii) a mode wherein the polarization inverted domain spreads from thecontact area of the first electrode to a surrounding area, and thepolarization inverted domains are not joined with each other butindependently present,

(iii) a mode wherein the polarization inverted domain spreads from thecontact area of the first electrode to a surrounding area, and partialareas of the polarization inverted domain are joined with each other,and

(iv) a mode wherein the inverted domain spreads from the contact area ofthe first electrode to a surrounding area, the inverted domains arejoined with each other but an area free of polarization-inversionremains.

(4) The production method of the above-mentioned (1), wherein aninsulation film is formed on one plate face of the nonlinear opticalcrystal substrate, an opening having a shape of the above-mentionedcontact area is formed in the insulation film to expose the plate facewithin the opening, and an electrode is contacted with the exposedplate, which is used as the first electrode.

(5) The production method of the above-mentioned (1), wherein two kindsof stripe insulation films different from each other in at least thelongitudinal direction of these bands are layered intersectionally onone plate face of the nonlinear optical crystal substrate, an exposedarea surrounded by belts of these two kinds of stripe insulation filmsis used as the above-mentioned contact area, and an electrode iscontacted with said area to give the first electrode.

(6) The production method of the above-mentioned (1), wherein theabove-mentioned contact area has a shape of a circle, an ellipse or apolygon with round corners.

(7) The production method of the above-mentioned (1), wherein a gapbetween the adjacent areas from the plural contact areas present in thedomain to be polarization-inverted is not more than 5 μm under theabove-mentioned conditions (A).

(8) The production method of the above-mentioned (1), wherein thenonlinear optical crystal substrate is a crystal substrate which is socut that the substrate contains the Y crystal axis in its main face, andthe plural contact areas present in the domain to bepolarization-inverted are arranged to lie in continuance in the Y axisdirection under the above-mentioned conditions (A).

(9) The production method of the above-mentioned (1), wherein thenonlinear optical crystal substrate is a crystal substrate made ofLiNbO₃, LiTaO₃, or LiNbO₃ or LiTaO₃ doped with other element.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram to explain the production method of thepresent invention and shows an embodiment wherein the whole domains tobe polarization-inverted are polarization-inverted and hatching isapplied to distinguish domains.

FIG. 2 is a partially enlarged view showing one example of an embodimentwherein the domains to be polarization-inverted are partiallypolarization-inverted in the production method of the present invention.

FIG. 3 is a partially enlarged view showing another example of anembodiment wherein the domains to be polarization-inverted are partiallypolarization-inverted in the production method of the present invention.

FIG. 4 is a partially enlarged view showing another example of anembodiment wherein the domains to be polarization-inverted are partiallypolarization-inverted in the production method of the present invention.

FIG. 5 is a schematic diagram explaining the definition of the contactarea in the present invention.

FIG. 6 is a schematic diagram exemplarily showing the shape of thecontact areas and the arrangement patterns in the present invention.

FIG. 7 is a schematic diagram exemplarily showing the shape of thecontact areas and the arrangement patterns in the present invention.

FIG. 8 is a sectional view showing the structure of an electrodesatisfying the conditions of the contact area in the production methodof the present invention.

FIG. 9 is a perspective view showing a preferable embodiment whereinopenings are formed using an insulation film in the production method ofthe present invention.

FIG. 10 is a graph showing the relationship between the size of adot-like contact area and the standard deviation of inversion ratios inExamples of the present invention.

FIG. 11 is a graph showing the dispersion in inversion ratios of FIG. 10based on conversion efficiency.

In FIG. 12, FIG. 12(a) is a microscopic photograph showing anarrangement pattern of openings in the contact area formed in aninsulation film in Example 2 of the present invention, and FIG. 12(b) isa microscopic photograph showing a polarization inverted structureobtained by the contact area in FIG. 12(a).

FIG. 13 is a microscopic photograph showing an end of the individualpolarization inverted domains in the polarization inverted structureshown in FIG. 12(b).

FIG. 14 is a microscopic photograph showing an arrangement pattern ofopenings in the contact area formed in an insulation film in Example 3of the present invention.

FIG. 15 is a perspective view schematically exemplifying the appearanceof a polarization inverted crystal.

FIG. 16 is a sectional view showing an example of an electrode structureformed to apply a polarization inverted electrode in a production methodof a conventional polarization inverted crystal.

FIG. 17 is a sectional view showing spread of the inside of the crystalof a polarization inverted domain formed by a conventional productionmethod.

FIG. 18 shows spread of the polarization inverted domain on thesubstrate surface when polarization-inversion was performed by aconventional production method.

The symbols in the above-mentioned Figures each show the followingconstituent elements. 1: nonlinear optical crystal substrate, 2: domainto be polarization-inverted (predetermined domain), 3: contact area, 4:polarization inverted domain.

BEST MODE FOR EMBODYING THE INVENTION

In the following, a polarization inverted domain intended to be finallyobtained in design or polarization inverted domain to be wholly invertedin design (i.e., domain to be polarization-inverted) is referred to as a“predetermined domain”, the first electrode to be disposed within thepredetermined domain is referred to as an “upper electrode” or “thefirst electrode”, and a second electrode to be disposed on the otherface of the crystal substrate is referred to as a “lower electrode” or“the second electrode”, in the explanation of the present invention.

FIG. 1 (a) shows a part of the plate face of a crystal substrate 1before forming a polarization inverted structure, wherein predetermineddomains 2 are drawn with a dotted line. In a polarization invertedstructure, plural band-like predetermined domains 2 are arranged on acrystal substrate face in parallel to each other, thus forming a stripepolarization inverted structure as a whole, which comprises apredetermined domain 2 to be polarization-inverted and a polarizationnon-inverted are alternately arranged.

In the production method of the present invention, the first electrodeis first brought into contact with the inside of each predetermineddomain 2 in a plate face (main surface) of a crystal substrate 1. Themode of contact area between the plate face and the first electrode atthis time is important. As in one example shown in FIG. 1(b), in thepresent invention, the first electrode is broken to give small dot-likecontact areas 3 that are dispersed like a mosaic within a predetermineddomain, and the electrode is formed, which comprises these dot-likecontact areas 3 collectively forming a predetermined domain 2 as a wholelike a pointillistic drawing. Particularly, the definition of the sizeof each dot-like contact area 3 here is important, and the definitioncorresponds to the above-mentioned conditions (A). The above-mentionedconditions (A) are described in detail below.

Then, a second electrode as a lower electrode is contacted with theother plate face of a crystal substrate, and a polarization inversionvoltage is applied. By the application of a polarization inversionvoltage, polarization-inversion occurs in the above-mentioned dot-likecontact areas and a production object of polarization inverted crystalis formed.

At this time, by selecting the shape of dot-like contact area,arrangement pattern, strength of polarization inversion voltage, andapplication time, a predetermined domain may take various modes as shownbelow, such as entirely polarization-inverted domain, assembly of smallpolarization-inverted domains and the like.

The above-mentioned mode (2) affords polarization inversion of thepredetermined domain in its entirety. In this case, by appropriatelyselecting the shape of the dot-like contact area and arrangement pattern(mentioned below), and continuing the application of a polarizationinversion voltage, polarization inverted domains spread from individualdot-like contact areas, are joined with each other, and finallyintegrated as shown in FIG. 1(c). At the time point when almost thewhole predetermined domain 2 is filled with a polarization inverteddomain 4, the object polarization inverted crystal is obtained.

In the above-mentioned mode (3), at least dot-like contact areas arepolarization-inverted but the entirety of the predetermined domain isnot completely polarization inverted. This mode can be said to be a modewherein the predetermined domain is partially polarization-inverted likea pointillistic drawing by terminating the polarization-inversion priorto reaching the above-mentioned mode (2). In this case, the mode ofpartial polarization inversion falls under any of the above-mentionedmodes (i)-(iv) by appropriately selecting the shape of the dot-likecontact area and arrangement pattern (mentioned below), and further byselecting the application time of a polarization inversion voltage.These modes are explained using an enlarged view taking a rectangularcontact area as an example. The arrangement pattern of the contact areasin the predetermined domain is as exemplarily shown in FIG. 1(b).

As shown in enlarged view of FIG. 2(a), the above-mentioned mode (i)comprises polarization inversion of a domain (solid line) 4 almost equalto a contact area 3 (dotted line).

Since polarization inversion tends to partially develop in a contactarea and spread to the surrounding area, it is difficult to form apolarization inverted domain completely identical to the contact area.However, any polarization inverted domain having a shape considered tobe generally equal to the contact area within the range of productionerror is within this mode. From the view of the entire predetermineddomain after completion of the polarization inversion, it is appreciatedthat small polarization inverted domains 4 collectively constitute onepseudo polarization inverted domain (shape equal to predetermined domain2) as a pointillistic drawing, as shown in FIG. 2(b).

In the above-mentioned mode (ii), individual contact areas arecompletely polarization-inverted and then polarization inversion furtherspreads outside the contact area, wherein polarization inverted domainsspread from the neighboring contact areas are independently presentwithout joining with each other (not shown). An assembly equivalent tothe assembly of polarization inverted domains obtained by this mode (ii)can be obtained by enlarging, in the above-mentioned mode (i), theindividual contact areas and narrowing the gap between them. Which modeis employed to obtain a similar final product can be determined inconsideration of processing precision of the electrode, management ofapplication time and the like.

In the above-mentioned mode (iii), polarization inverted domains spreadfrom the contact areas 3 of the first electrode as shown in FIG. 3(a) tothe outside, and the regions located near in the polarization inverteddomains are joined, as shown in FIG. 3(b). The arrangement pattern ofthe contact areas in this mode is one wherein the domains intended to bejoined (e.g., a pair of contact areas 3 a and 3 b) are brought close toeach other, as shown in FIG. 3(a). From the view of the wholepredetermined domain after completion of the polarization inversion,while individual polarization inverted domains 4 become large becausethe domains are joined, small polarization inverted domains 4collectively constitute one pseudo polarization inverted domain (shapeequal to predetermined domain 2) as a pointillistic drawing, as shown inFIG. 2(b).

In the above-mentioned mode (iv), the polarization inverted domainspreads from the contact areas 3 of the first electrode as shown in FIG.4(a) to the surrounding area, and all or the most part of thepolarization inverted domains 4 are connected to become one but domains4 b free of polarization inversion remain, as shown in FIG. 4(b).

In the above-mentioned modes (i)-(iv), polarization inversion is stoppedbefore polarization inverted domains that spread from the dot-likecontact areas to the outside are joined with each other and completelyfill the predetermined domain. Seen from the whole predetermined domain,entire inversion has not occurred. As compared to entire inversion ofthe predetermined domain, however, the dispersion in the total area ofthe inverted domain in the predetermined domain can be minimized. Thismeans a smaller dispersion in the ratio (inversion ratio) of theinverted domain and the non-inverted domain when seen from the wholeperiodic polarization inverted structure (alternate arrangementstructure of inverted domain and non-inverted domain), and possibleachievement of more highly efficient quasi-phase matching. In addition,the dispersion of the quality of products becomes small and polarizationinverted crystals having stable quality can be always obtained. Takingnote of the action of the whole periodically polarization invertedstructure, even if individual inversion domains are assemblies of smallpolarization inverted domains, they act on the propagated light as ifthe whole predetermined domains were polarization-inverted, thusproviding quasi-phase matching comparable to conventional matching.

Using first electrode as an assembly of dot-like contact areas anddispersing them within the predetermined domain like a pointillisticdrawing, almost the same edge effect is seen in any contact area, andinversion starts simultaneously from every contact area. In other words,when the whole predetermined domain is macroscopically seen,inconsistent phenomenon of early start of inversion depending on theposition of the predetermined domain does not occur unlike conventionalcases, and the following action and effect can be provided.

(a) An incident of preceding inversion of both ends alone does notoccur, and even if the predetermined domain occupies a wide area,uniform polarization inverted domain is obtained in a short time sinceinversion starts simultaneously from each point. In addition, since theinversion completes in a short time, the part where inversion is startedearlier does not spread in the period direction, unlike conventionalcases.

(b) Even if the predetermined domain is variously changed in size, asimilar inverted domain is always obtained by making the above-mentionedconditions (A) always the same, specific conditions for every kind ofpolarization inverted structure do not need to be set.

When a predetermined domain is polarization-inverted in its entirety, itis preferable that the predetermined domain 2 in FIG. 1(a) and the finalpolarization inverted domain 4 in FIG. 1(c) as a final productcompletely match. However, over and under inversions of thepredetermined domain may be present as long as they are within thenecessary quality range or the range of acceptable production error.

The above-mentioned conditions (A) define the presence of contact areain individual predetermined domains and can be divided into thefollowing (A1) and (A2).

(A1) Plural dot-like contact areas are independently present like amosaic in individual predetermined domains, and dispersed to form apredetermined domain as a whole like a pointillistic drawing.

(A2) Individual dot-like contact areas have an area of 0.00785 μm²-7850μm² and a shape included in a circle with a diameter of 100 μm.

The above-mentioned conditions (A1) define dispersing dot-like contactareas in a predetermined domain, but do not limit that the individualcontact areas are completely independent small areas having no contactpoint with each other.

For example, even if the main parts (square) 3 a of the dot-like contactareas are connected with each other by conductive paths 3 b as shown inFIG. 5, such ultrafine conductive paths are ignored and the contactareas are considered to be substantially dispersed in a dot-like state,as long as the main parts 3 a each satisfies the above-mentionedconditions (A2) and affords the above-mentioned action and effect of thepresent invention.

The above-mentioned conditions (A2) define the area and shape ofindividual contact areas such that they show a particular dot-like statecapable of affording the above-mentioned action and effect. Limitationonly on the area includes an excessively extended long shape. Such longshape causes polarization inversion only in both ends due to the edgeeffect similar to that mentioned in Prior Art, and inhibits the actionand effect of the present invention. To prevent the contact area frombeing excessively deformed in a long shape, the deformation was limitedto not exceed a circle having a particular diameter.

The individual contact areas have an area of 0.00785 μm²-7850 μm². Whenthe area exceeds this range, individual contact areas become too largeto realize an arrangement wherein small dot-like contact areas aregathered like a mosaic to draw a predetermined domain as a whole, andthe above-mentioned action and effect cannot be achieved. When the areais less than this range, the contact areas are too small to be formed,and inversion cannot be performed. Of the above-mentioned area range, 1μm²-100 μm² is particularly an optimal area to be dispersed inindividual domains actually formed in a polarization inverted structure.

The shape of the individual contact areas may be any as long as theyhave the above-mentioned area and are included in a circle having adiameter of 100 μm, which exhibits the action and effect afforded bydividing into dots. The “included in a circle” contains being inscribedin the circle and equal to the circle.

In consideration of the size of the inverted domain in the polarizationinverted structure actually used as well as unpreferable excessivelylong shape of a dot (inversion starts from both ends that areexcessively apart), the diameter of the aforementioned circle is furtherlimited and 50 μm, particularly about 10 μm, is preferable.

The shape of the contact areas is preferably square, rectangle,circular, ellipse (including oval having semicircle areas on both endsof a strip region in the longitudinal direction and deformed circle),equilateral triangle and the like, because the spread of polarizationinversion from these shapes to the outside can be easily calculated.However, the shape may be any and is not limited to these, and may bepolygon, star and the like. In addition, it may be a shape of polygon orany shape having round corners.

As explained in Prior Art, when the whole polarization inverted domainis seen, both ends of the band domain are problematically inverted dueto an edge effect. When small contact areas are noted, conversely,pointed parts such as corners of polygon and the like are sometimesuseful for the formation of an initial nucleus for inversion.

When dot-like contact areas are arranged in individual predetermineddomains, the gap between the adjacent contact areas (namely, minimumdistance) is preferably not more than 5 μm, by which the inverteddomains are integrated in a short time.

As shown in FIG. 3, to join only the polarization inverted domainsdeveloped from particular contact areas (pair of 3 a, 3 b and the like),the gap between the domains desired to be joined alone only needs to beset narrower than the gap between the domains not intended to be joined,for example, not more than 5 μm and the like.

No limitation is imposed on which contact area having what shape is tobe disposed in individual predetermined domains in what arrangementpattern, and the above-mentioned embodiments can be freely combinedwithin the range satisfying the above-mentioned conditions (A).

For example, in FIG. 6(a), square contact areas 3 are arranged regularlyto form a matrix, and in FIG. 6(b), they are arranged by a half-pitchstaggered for each row as in bricklaying. In FIG. 7(a), circular (or maybe equilateral triangular) contact areas are densely arranged.

Of the above-mentioned combination embodiments, a combination whereinthe contact areas are circles having a diameter of about 1 μm-2 μm,which are arranged to form a matrix pattern within a predetermineddomain, and the total area is about 20%-60% of the area of thepredetermined domain is particularly preferable because thepredetermined domains can be simultaneously inverted.

When the band width (length in period direction) of the predetermineddomains is narrow, dot-like contact areas form an arrangement pattern inone row in the longitudinal direction of the band, as shown in FIG.7(b). In this case, too, as long as the above-mentioned conditions (A)are satisfied, the pattern may comprise contact areas having the sameshape, which are arranged at a regular pitch, or contact areas havingvarious shapes, which are arranged at irregular pitches.

When dot-like contact areas are dispersed in a predetermined domain, asuitable gap t1 (space free of contact area) suitable for the density isformed between the outer boundary of a predetermined domain and contactareas, whereby overflow of polarization inversion from a predetermineddomain is preferably suppressed.

Since the inverted domain tends to spread beyond the contact area by notless than 1 μm², the aforementioned gap t1 is preferably not less than 1μm.

The aforementioned gap t1 may be determined in association with the gapt2 between the adjacent contact areas, herein t1 is preferably a half oft2.

To form dot-like contact areas, a number of electrodes having a dot-likearea are used and brought into contact with the crystal substrate face(not shown). In a preferable embodiment of an electrode, an insulationfilm R is formed on a plate face of a crystal substrate 1, and thenopenings 3 a having the shape of the above-mentioned contact area isformed on the insulation film R to expose a plate face in the openings 3a, as shown in FIG. 8(a), and an electrode 5 is contacted with theexposed part to give a first electrode, as shown in FIG. 8(b), can bementioned.

In the embodiment shown in FIG. 8(b), the whole insulation film Rcontaining openings 3 a is covered with an electrode layer 5, thusallowing contact of the electrode layer 5 with a dot-like crystalsubstrate exposed in the opening, whereby a dot-like contact area isobtained. For current-carrying to the electrode layer 5, moreover, aliquid electrode (electrolyte) 6 is brought into contact with the topsurface of the electrode layer 5. The electric field produced in thecrystal substrate by the application of a voltage is shown with anarrow.

As a material for the insulating layer, known materials such as PMMA(polymethacrylic acid methyl) and the like can be used and, as a formingmethod, film forming methods such as spin coat and the like can be used.

When patterning is necessary for the formation of an insulating layer,an electrode and the like, known patterning techniques such asphotolithography and the like can be used.

For the formation of an electrode layer (metal electrode film), knownfilm forming methods such as sputtering, electron beam vapor depositionand the like can be used.

As the liquid electrode, liquid electrolytes used for known liquidelectrode methods, liquid metals such as gallium, indium, mercury andthe like, and the like can be used.

As a solvent constituting a liquid electrolyte, water, polyol, a mixtureof these, and the like can be mentioned. As a material of anelectrolyte, lithium chloride, sodium chloride, potassium chloride andthe like can be mentioned.

Moreover, for a container necessary for contacting a liquid electrodewith the top surface of a metal electrode or a back face of a crystalsubstrate, a wire structure to be connected to a liquid electrode, andan electric power-supply unit (including control circuit and the like),those used for known liquid electrode methods can be used.

Polarization inversion is easily developed when the electric fieldacting on crystal is high, and tends to spread toward the surroundingarea. An embodiment shown in FIG. 8(b) is considered, wherein the entireinsulation film R including openings 3 a is covered with an electrodelayer 5, and the insulation film has a thin part and a thick part. Inthis case, the electric field acting on the crystal becomes higher inthe part where the insulation film is thin than in the part where it isthick, and the polarization inverted part tends to spread beyond thecontact areas.

Therefore, in an embodiment where an opening is formed in an insulationfilm and covered with a conductor, as in FIG. 8, the thickness of theinsulation film is preferably such that the thickness of the insulationfilm between electrodes in the connecting direction of inverted domainsis thinner than the thickness of other insulation films. As a result,the inverted domains spread with directional movement and only thedomains to be connected are easily connected.

As a method of making an insulation film topically thin as mentionedabove, for example, when a uniform photoresist is used, a resist layerbetween electrodes can be made thin by controlling the amount of UVexposure in a photolithography step. The amount of UV exposure can becontrolled not only by changing the size intensity of light source butalso by making the thickness variation of a photomask pattern.

A photomask limits the amount of UV radiation to the resist, and whenthe light-shielded part of a photomask is semitransparent (i.e., pale),that part of the resist is exposed to light to some extent, and whendeveloped, that part of the resist is not completely removed but remainsthin. In addition, an effect equivalent to the effect provided by makinga thin resist can be provided by forming a photomask having a finepattern (e.g., stripe, net, coil, free line pattern, dot-like and thelike can be mentioned but not limited thereto) of not more than thepatterning resolution, on a part to be topically made thin.

Another preferable embodiment wherein an opening is formed using aninsulation film as mentioned above is explained below. FIG. 9 shows oneexample of the embodiment, wherein two kinds of stripe insulation filmsR1, R2 different from each other in at least the longitudinal directionof the band are layered intersectionally on one plate face of a crystalsubstrate 1. Exposed rectangular contact areas 3 are formed bysurrounding four sides thereof with the bands R1, R2 of these two kindsof stripe insulation films, and an electrode is contacted with thedomains 3 in the same manner as in FIG. 8 to give a first electrode.

As a production method of the two layer insulation film shown in FIG. 9,for example, a method wherein one stripe insulation film R1 is a layermade of SiO₂, the other stripe insulation film R2 is a photoresistlayer, the insulation film R1 is vapor deposited on the substratesurface, a stripe structure is formed by photolithography, etchingprocessing, and then the photoresist layer of the insulation film R2 ispatterned by photolithography, and the like can be mentioned.

According to such production method, the thickness of each layer can beindependently controlled freely since production conditions for eachlayer are independent, and a mask layer having different thickness forevery direction can be easily produced. When an equivalent structure isto be produced with a single layer, photomask processing or patterningconditions of photolithography is/are limited, making productiondifficult.

In an embodiment using a two-layer stripe insulation film as shown inFIG. 9, the direction of the two stripes may be any, but it ispreferable to make them mutually perpendicular to give quadrel contactareas, and to match the direction of one stripe (i.e., direction of oneside of quadrel contact area) with the longitudinal direction of theband of the predetermined domain.

The nonlinear optical crystal may be known. For example, representativeones such as LiNbO₃, LiTaO₃, X_(A)TiOX_(B)O₄ (X_(A)=K, Rb, Tl, CS,X_(B)=P, As) and the like, and these doped with various other elementssuch as Mg and the like can be mentioned. LiNbO₃ and LiTaO₃ may becongruent compositions or stoichiometric compositions.

Ferro-electric crystals such as LiNbO₃, LiTaO₃ and the like have beenpreferably used as materials of the elements to be subjected towavelength conversion, such as second harmonic generation, OpticalParametric Oscillation or amplification, Difference FrequencyGeneration, Sum Frequency Generation and the like. MgO-doped LiNbO₃ is amaterial particularly superior in the resistance to optical damage.

The crystal substrate to be subjected to polarization inversionprocessing is representatively a z plate. It may be an off-cut platewherein a particular crystal axis forms a particular angle (off angle)with the normal line of the substrate surface. A z plate is a crystalsubstrate so cut that the direction of the Z axis of the crystal isperpendicular to the substrate surface (i.e., Z-cut). A crystalsubstrate is preferably a single domain (single polarization treatment)in its entirety having the same polarization direction.

When a crystal substrate which has been so cut that the Y axis of thecrystal axis is contained in the main surface of a crystal substrate, asrepresented by Z plate, is used, in one of the preferable embodiments,plural contact areas present in the domain to be polarization-invertedare disposed to lie in continuance in the Y axis direction. Sinceferro-electric crystals such as LiNbO₃, LiTaO₃ and the like (includingthese doped with impurity such as MgO and the like) show easy growth ofan inverted domain in the Y axis direction, a predetermined band domainis easily formed by disposing, on the surface of the crystal substratesuch as Z plate and the like, contact areas to lie in continuance in theY axis direction.

While the size of a crystal substrate is not limited, as an exemplarysize of a cuboid plate, the length in the light path direction is about5 mm-70 mm, the size of the section perpendicular to the light pathdirection is about (3 mm×70 mm)-(0.2 mm×5 mm). A polarization invertedcrystal formed in such size can be used as it is or after freelydividing or processing.

In the wavelength conversion according to quasi-phase matching methodsusing a polarization inverted structure, maximum conversion efficiency(m=1) depends on a polarization inversion ratio D, wherein the mostideal value is D=1/2, namely, when the band width (domain width) of apolarization inverted domain is half the polarization inversion period,the conversion efficiency is highest as a wavelength conversion element.Thus, when a periodic polarization inverted structure is to be produced,what is most important is to produce every polarization inverted domaindisposed in a stripe, such that every domain shows a polarizationinversion ratio of 50% with uniform precision.

According to the production method of the present invention, due to thegeneration of polarization inverted domain from contact areas dispersedand gathered like a pointillistic drawing, any polarization inverteddomain and any part of individual polarization inverted domains becomeuniform.

The voltage to be applied for polarization inversion may be determinedby reference to known polarization inversion techniques. For example, amethod comprising application of a direct voltage for a given time, anda method comprising application of a pulse voltage can be mentioned. Avoltage is applied in the direction that makes the potential relativelypositive in the +z-plane and relatively negative in the −z-plane.Particularly, the action and effect of the dot-like electrodes dispersedaccording to the production method of the present invention become mostremarkable when a pulse voltage is applied. Application of the pulsevoltage facilitates simultaneous inversion of the entire domains, andimproves content uniformity of the inverted structure.

EXAMPLES

The production method of the present invention is described in detail inthe following by showing Examples.

Example 1

In this Example, the shape of the dot-like contact area was ellipse(oval wherein both ends in the longitudinal direction of a band domainare each a semicircle, which can be also said to be an oval obtained bymaking round corners of a rectangle), and band-like polarizationinverted domains had various sizes in the longitudinal direction. Eachsample was subjected to polarization inversion.

As a crystal substrate, a 0.3 mm-thick Z cut LiNbO₃ (hereinafter to beindicated as “MgO:LN”) containing MgO by 5 mol % was used.

As to the design specification of the polarization inverted structure tobe produced, the size of a polarization inverted structure as a whole ona crystal substrate surface was structure width (longitudinal directionof stripe) 1 mm, length of structure in the light path direction 15 mm,polarization inversion period 7.0 μm, and inversion ratio 50%.

Thus, the shape of the individual predetermined domains was a rectanglewherein the length of the stripe in the longitudinal direction(longitudinal direction of band) was 1 mm and the length in the perioddirection (band width direction) was 3.5 μm.

MgO: An insulation film was formed on a +z-plane of an LN substrate andan insulation film having dot-like holes was formed with photoresist.

The shape of the holes to be the dot-like contact areas was ellipse(oval) as mentioned above, and respective samples having a length in thelongitudinal direction of band ranging from 1.0 μm as the minimum sizeup to 500 μm were produced. The length of the contact area in the perioddirection of the stripe (band width direction) was constantly 2.0 μm,and the gap between the adjacent contact areas in the same predetermineddomain was constantly 1 μm.

A metal film was formed on the entire surface of the insulation film bysputtering (lower layer Cr/upper layer Au) and the electrode membranewas also contacted with the substrate surface exposed in the opening.

A pulse voltage was applied between substrate ±Z planes using an LiClelectrolyte solution to invert the dot portion where the metal film wasin contact with the substrate.

The metal film and photoresist were removed, selective etching wasperformed using fluoro-nitric acid and the polarization invertedstructure was observed from the substrate surface and the substratesection. As a result, it was confirmed that a uniform periodic invertedstructure had been formed over the entire area of the structure of everysample.

In addition, the relationship between the size of the dot-like contactarea and the polarization inversion ratio (polarization inversionwidth/polarization inversion period) was evaluated for theabove-mentioned samples. For comparison of the inversion shapedistribution, dispersion of the inversion ratios of the +z-plane and the−z-plane was expressed by the standard deviation value. The results areshown in FIG. 10, graphs (a), (b) by plotting with black circles.

As is clear from the graph of FIG. 10, the standard deviation becameexponentially smaller with decreasing dot-like contact areas, resultingin smaller variation. In addition, it was found that the variation wasgreater in the −z-plane which is a uniform electrode.

The dispersion of the inversion ratio causes lower conversionefficiency. The dispersion of the inversion ratio as shown in the graphsof FIG. 10(a), (b) was converted to conversion efficiency (conversionefficiency in the absence of dispersion is 100%), and shown in thegraphs of FIG. 11(a), (b) by plotting with black circles. As is clearfrom the graphs in this Figure, an inflection point relating to thehigh-low of conversion efficiency was present around the dot size ofabout 100 μm.

As shown in FIG. 11(b), it is clear that the size of the dot affordingnot less than 80% of the conversion efficiency is not more than 100 μmin the −z-plane where the conversion efficiency is generally low.

Example 2

In this Example, a 0.5 mm-thick Z cut MgO:LN containing MgO by 5 mol %was used as a crystal substrate.

As to the design specification of the polarization inverted structure tobe produced, the size of a polarization inverted structure as a whole ona crystal substrate surface was longitudinal direction 10 mm, the lightpath direction 30 mm, period 30 μm, and diameter of dot-like contactarea 2 μm.

The arrangement pattern of the contact areas was a matrix, as shown inFIG. 12(a). The designed width of the predetermined domain in the perioddirection was 15 μm, and a space (area free of contact area) of 2 μm wasformed on both ends. The areas were disposed in the center of a bandwidth of 11 μm with a gap between the contact areas of 1 μm (i.e.,center to center pitch 3 μl).

Polarization inversion was performed in the same manner as in Example 1,and the obtained polarization inverted structure was observed. As aresult, it was confirmed that a uniform periodically inverted structurehad been formed over the entire area of the polarization invertedstructure, as shown in FIG. 12(b).

In addition, the end of each inversed domain (band-like) of the obtainedpolarization inverted structure was observed. As shown in FIG. 13(a)providing a microscopic photograph of the +z-plane side of the end andFIG. 13(b) providing a microscopic photograph of the −z-plane side ofthe end, the edge effect was found to have been preferably suppressed toobliterate overflow from the predetermined domain.

Comparative Example 1

In the same manner as in the above-mentioned Example 2, a polarizationinverted structure having the same specification as in theabove-mentioned Example 2 was formed according to a conventional methodexcept that a metal electrode was brought into contact with the entiresurface of the predetermined domain, and the quality was compared.

First, as regards the inversion width, it was wider by 3-4 μm inComparative Example than the electrode width, but the one obtained inExample 2 showed a suppressed width of 1 μm.

Furthermore, the dispersion in polarization inversion ratios (inversionwidth/inversion period) was compared in terms of standard deviationvalue. As a result, the product of Comparative Example showed dispersionof 9%-10% (2.2 μm-2.4 μm in width) in the +z-plane, and 12%-14% (2.9μm-3.4 μm in width) in the −z-plane. The product obtained in Example 2showed dispersion of 4% in the +z-plane and 5% in the −z-plane, thusaffording markedly improved dispersion in the polarization inversionratio.

Example 3

In this Example, a crystal substrate similar to that in Example 1 wasused, and dot-like contact areas, which were circles having a diameterof 1 μm, were arranged to form one line at period 7.0 μm in thelongitudinal direction of the predetermined domain, as shown in FIG. 14.

By the process similar to that in Example 1, an electrode was formed inthe +z-plane of a substrate, and a voltage was applied to formpolarization inversion.

After the inversion, the inverted structure was confirmed by selectiveetching using fluoro-nitric acid. As a result, it was found that theuniformity of the inversion ratio had been markedly improved as comparedto conventional methods.

The dispersion in polarization inversion ratios (inversionwidth/inversion period) was compared in terms of standard deviationvalue. As a result, the product produced by conventional methods showeddispersion of 11.9% (0.83 μm in inversion width) in the +z-plane, and15% (1.05 μm in inversion width) in the −z-plane. The product obtainedin this Example showed dispersion of 4% in the +z-plane (0.28 μm ininversion width), and 5% in the −z-plane (0.35 μm in inversion width),thus affording markedly improved dispersion.

Example 4

In this Example, a crystal substrate similar to that in Example 1 wasused and, as the design specification of the polarization invertedstructure to be produced, the size of a polarization inverted structureas a whole on a crystal substrate surface was longitudinal direction 10mm, the light path direction 50 mm, period 18 μm, and dot-like contactareas (square with one side 7 μm). In the same manner as in Example 1,polarization inversion was performed.

After the inversion, the inverted structure was confirmed by selectiveetching using fluoro-nitric acid. As a result, it was found that theuniformity of the inversion ratio had been improved as compared toconventional methods.

Example 5

In this Example, in a polarization inversion test under the sameconditions as in Example 1, the size of the contact area (ellipse) inthe longitudinal direction was 25 μm and the application of voltage wasstopped in the polarization inversion step before polarization inverteddomains that spread from the neighboring contact areas were bonded toeach other.

The size of the gap (non-inverted part) left between polarizationinverted domains was 1.5 μm on average.

The dispersion in the inversion ratio was examined along the light pathdirection of the inverted domain alone, while ignoring the part free ofbonding. As a result, the dispersion was 4.1% for the +z-planecontaining contact areas and 5.8% for the −z-plane on the back. Theresults are shown in the graphs of FIG. 10(a), (b) by plotting withwhile circles.

In contrast, in the sample of Example 1 wherein a voltage wascontinuously applied until the inverted domains were integrated, and thewhole predetermined domain was polarization-inverted. The dispersion inthe inversion ratio along the light path direction was 6.7% for +z-planeand 8.6% for the −z-plane on the back.

From these results, it was found that dispersion of inversion ratio wassuppressed in the embodiment of the above-mentioned (3), wherein theinversion was stopped before completion of the polarization-inversion ofthe entire inner surface of the predetermined domain, to about half thatin the embodiment of the above-mentioned (2), wherein the entire innersurface of the predetermined domain was polarization-inverted.

INDUSTRIAL APPLICABILITY

As explained above, according to the production method of the presentinvention, a high quality polarization inverted crystal can be obtainedmore easily.

This application is based on a patent application No. 20 03-070802 filedin Japan, the contents of which are hereby inco rporated by reference.

1. A production method of a polarization inverted crystal, whichcomprises a step of bringing a first electrode into partial contact withdomain(s) to be polarization-inverted, which domains are present in thenumber of not less than 1 in one plate face of a nonlinear opticalcrystal substrate, bringing a second electrode into contact with theother plate face of the substrate, and applying a polarization inversionvoltage between the both electrodes, wherein, in the aforementionedstep, the electrodes are so formed that the contact area of the firstelectrode relative to the plate face satisfies the conditions of thefollowing (A), and the domain to be polarization-inverted is entirely orpartially polarization-inverted by the application of a polarizationinversion voltage: (A) respective contact areas are dispersed like dotsin individual domains to be polarization-inverted such that pluralcontact areas are independently present, and individual dot-like contactareas have an area of 0.00785 μm²-7850 μm² and a shape included in acircle with a diameter of 100 μm.
 2. The production method of claim 1,wherein the domain to be polarization-inverted is entirelypolarization-inverted by the application of a polarization inversionvoltage.
 3. The production method of claim 1, wherein the domain to bepolarization-inverted is partially polarization-inverted by theapplication of a polarization inversion voltage and the partialpolarization inversion is any of the modes of the following (i)-(iv):(i) a mode wherein an area about the same as the contact area of thefirst electrode is polarization-inverted, (ii) a mode wherein thepolarization inverted domain spreads from the contact area of the firstelectrode to a surrounding area, and the polarization inverted domainsare not joined with each other but independently present, (iii) a modewherein the polarization inverted domain spreads from the contact areaof the first electrode to a surrounding area, and partial areas of thepolarization inverted domain are joined with each other, and (iv) a modewherein the polarization inverted domain spreads from the contact areaof the first electrode to a surrounding area, the polarization inverteddomains are joined with each other but an area free ofpolarization-inversion remains.
 4. The production method of claim 1,wherein an insulation film is formed on one plate face of the nonlinearoptical crystal substrate, an opening having a shape of said contactarea is formed in the insulation film to expose the plate face withinthe opening, and an electrode is contacted with the exposed plate, whichis used as the first electrode.
 5. The production method of claim 1,wherein two kinds of stripe insulation films different from each otherin at least the longitudinal direction of these bands are layeredintersectionally on one plate face of the nonlinear optical crystalsubstrate, an exposed area surrounded by belts of these two kinds ofstripe insulation films is used as said contact area, and an electrodeis contacted with said area to give the first electrode.
 6. Theproduction method of claim 1, wherein said contact area has a shape of acircle, an ellipse or a polygon with round corners.
 7. The productionmethod of claim 1, wherein a gap between the adjacent areas from theplural contact areas present in the domain to be polarization-invertedis not more than 5 μm under said conditions (A).
 8. The productionmethod of claim 1, wherein the nonlinear optical crystal substrate is acrystal substrate which is so cut that the substrate contains the Ycrystal axis in its main face, and the plural contact areas present inthe domain to be polarization-inverted are arranged to lie incontinuance in the Y axis direction under said conditions (A).
 9. Theproduction method of claim 1, wherein the nonlinear optical crystalsubstrate is a crystal substrate made of LiNbO₃, LiTaO₃, or LiNbO₃ orLiTaO₃ doped with other element.